The storage of digital or analog information on rotating disk media is well known. Particularly common, in data processing applications, is the magnetic disk file in which information is written on and read from concentric tracks on the disks by electromagnetic transducing heads supported adjacent the disk surfaces. At typical state-of-the-art track densities of, say, 10 tracks/mm, such a disk file must be provided with position reference information which is employed by a head positioning servo system to position and maintain the head precisely over a selected track of an associated disk. The operation of maintaining the head over a desired track is known as "track following" whereas that of moving the head between tracks is known as "track accessing". Both these operations make use of such position reference information.
In some disk files, position reference information is provided remotely from the disk surface on which the data to be processed is stored e.g. on a dedicated servo disk or disk surface. A general description of a disk file employing this type of head positioning system is given in an article entitled "Design of a Disk-File Head-Positioning Servo" by R. K. Oswald (IBM J. Res. Develop., September 1974, p 506). This type of system has the advantage that position reference information is continuously available. However, at higher track densities, such an arrangement has the disadvantage that it is difficult to guarantee registration between the remote position reference information and the information storage tracks of the disk.
To overcome this disadvantage, it is also known to provide position reference information in sectors, known as "servo sectors", on the information storage surface. These servo sectors are interspersed with "data sectors" containing the stored information and provide accurately registered position reference information on a sampled basis as the disk rotates. UK patent No. 1,314,695 entitled "Position Control System" describes such a system.
Both the dedicated servo system of the above referenced article and the sampled system of the referenced patent employ the common principle that the position reference information contains contiguous servo tracks of two alternating types whose boundaries each, nominally, coincide with the centre of a data track. Signal contributions from each type of servo track, as detected by the transducing head, are inherently distinguishable from each other. Demodulating circuitry separates these components from each other and derives a position error signal from the difference in their amplitudes. This position error signal varies cyclically with radial displacement of the head across the tracks. It is, ideally, linear between slope reversals and is zero when the head lies equally over the boundary of an adjacent pair of servo tracks.
In practice, the simple difference, (p-q) between the contributions, p and q, from different servo tracks, is not an accurate indication of displacement from the servo track boundary because, for one reason, the flying height of the head is different at different radial positions over the disk and this affects the amplitude of the detected signals. To overcome this problem, both the above referenced documents propose that the sum (p+q) of the contributions from a pair of servo tracks also be determined separately. Since (p+q) corresponds to the full width response of the head, it is a constant quantity and may be used to normalise the amplitude of the position error signal. The value of the position error signal is thus given by (p-q)/(p+q) multiplied by a constant. This technique is effected by means of automatic gain control of a variable gain amplifier to which the signals detected by the head are applied. The gain control signal is derived from a comparison of (p+q) with a reference value and is fed back to control the gain of the amplifier.
An alternative arrangement of position reference information, known as a "null" pattern is shown in an article entitled "Null Servo Pattern" by A. J. Betts (IBM Technical Disclosure Bulletin, Vol. 18, No. 8, January 1976, p 2656). This pattern again employs alternating servo tracks but with time coincident transitions of opposite magnetic polarity. When a transducing head is located over the boundary between two such tracks, it is subject to equal and opposite magnetic flux changes so that the net output signal is zero. When the head is off-centre, the flux changes are not equal and a net error signal results. With this arrangement, the contributions of each track to the head output signal are not separately identifiable as they are combined in the head itself. Consequently, the response of the head cannot be normalised by means of signals derived from the position reference information. For this reason, a separate gain field of alternating unipolar transitions is provided, which precedes the position reference information. The signals detected by the head from the gain field represent the maximum amplitude response of the head to a transition and can thus be averaged and employed to normalise the subsequent position error signals.
Although the above described automatic gain control techniques will standardise the response of different heads to some extent, they do not do so adequately in cases where the widths of the heads vary. Such variations become more likely at high track densities because the width of the transducing head must be decreased to the extent that its dimensions are difficult to control. In the case of electromagnetic transducing heads, the effective width of the head may be greater than the physical gap width because of the effect of fringing fields. In these circumstances, to avoid crosstalk, it is important that the physical widths of all data heads are significantly less than the data track widths. Furthermore, for reasons elaborated in our European Patent Application "Magnetizable Recording Disk and Disk File employing Servo Sector Head Positioning" (A. J. Betts and P. J. Elliott), filed concurrently herewith, a deliberate choice of a head width which is only a fraction of the servo track pitch may be desirable. Particularly, where narrow data heads are used to read sector servo position reference information, large variations in off-track response can arise if the conventional AGC approach is employed. Such variations in position error signal slope for heads of different widths, to which conventional AGC has been applied, are illustrated in FIG. 11 and discussed in detail in the following section entitled "Disclosure of the Invention".
This problem has been recognised in the art, in the context of a dedicated servo file, and a solution is proposed in an article entitled "Off-track gain calibration of position error signal" by R. S. Palmer (IBM Technical Disclosure Bulletin Vol. 20, No. 1, June 1977, p 349). This article shows a servo pattern in which four radially overlapping but time separated servo signals "A, B, C and D" are employed, the contiguous boundaries of A and B and of C and D defining two data track centres. A conventional AGC circuit uses the sum A+B+C+D to control a variable gain amplifier. To adjust off-track gain, special calibration track portions are written in which signals A and B are separated by equal and opposite sinusoidally varying distances from the data track centre line. This affects the head response in the same way as a movement off track. The response is averaged and applied as a second control signal to the variable gain amplifier, thereby standardising the off track gain.
To compleFte the review of the prior art, reference is also made to UK patent No. 1,489,078 in which problems of asymmetry in magnetic heads or servo channels are overcome by separate adjustment of variable gain amplifiers in dual servo channels for demodulating the signals from respective contiguous servo tracks. The solution involves writing the servo tracks in a serpentine form so that their boundary defines a serpentine path. At normal disk operating speeds, the sinusoidal modulation of the head output signal is of such high frequency that it does not affect the normal track following operation which is handled by a lower bandwidth feedback loop employing separate channels for each of the two servo track types. The high frequency components, corresponding to the sinusoidal modulation, are filtered out from the separate channels by high frequency filters. The average amplitude of the high frequency filtered signal is employed to control the variable gain amplifier in each channel thereby overcoming problems of asymmetric response.